This invention relates to transducers, and modulators and sensor devices constructed therewith which are responsive to physical quantities such as pressure, temperature, etc., and, in particular, to such transducers and transducer devices utilizing fiber optics.
There are a variety of ways of altering the optical path length of an optical core in an optical waveguide, such as by stretching the waveguide or fiber to change its physical length, raising the index of refraction of the core material by raising the temperature, or even by altering the characteristics of regions just outside the core. Examples of practical applications of such optical path length altering methods may be found in the cross-referenced Ser. No. 549,875 and Ser. No. 241,861 patent applications. Indeed, much work has been done to build interferometric sensors in which one of the two arms of an optical fiber interferometer is made subject to an external field or other condition to be sensed. In the Ser. No. 549,875 and Ser. No. 241,861 applications, the present inventor has described a method of using a single optical core to carry both beams of the interferometer, each in a different polarization state. Also described were various waveguide structures, including complex internal architectures and hollow cavities, that can be used to magnify the effects of an external field or sensed condition, such as pressure, on the core and thereby enhance the sensitivity of the interferometer.
With the recent advances exemplified by the invention disclosed in U.S. patent application Ser. No. 331,052 for OPTICAL RESONANT CAVITY FILTERS, it is practical to utilize resonant cavity structures as transducers or sensors. There are several advantages to resonant cavity transducers.
For a given length, a resonant cavity is more sensitive than the two-armed interferometer, and the increased sensitivity is proportional to the "finesse" of the device. The "finesse" of an optical device is the ratio of the spacing between output peaks or spikes to the width of the peaks.
Secondly, a resonant cavity lends itself naturally to a method of determining the value of the perturbing parameter in which a narrow wavelength source is scanned by the sensor and peak shift is determined. The wavelength shift of the output looks much like the parameter dependence in contrast to interferometric devices where output typically varies in a sinusoidal manner with parameter variation.
A third advantage is that resonant cavities do not require the splitting of a source beam into two separate beams and the subsequent recombining of the beams at the interferometer output.
A fourth advantage is that a resonant cavity is relatively insensitive to input power changes in the beam, particularly in the wavelength scanning mode, so long as the power variation is slow compared to scanning time. On the other hand, the interpretation of an interferometer output is absolutely dependent on the power input unless wavelength scanning, a reference beam, or some equivalent is used.
The fifth advantage is that a resonant cavity device does not require a reference arm and, accordingly, the output of a resonant cavity device is not subject to errors in the reference beam, as can occur in an interferometer device.